Thermoelectric transducing material thin film, sensor device, and its manufacturing method

ABSTRACT

The present invention provides an SiGe-based thin film, a method for manufacturing this thin film, and applications of this thin film. The present invention relates to a method for producing, by sputtering, an SiGe-based semiconductor thin film to serve as a member of a thermoelectric transducing material component that is a constituent element of a sensor device whose signal source is a temperature differential and that transduces a local temperature differential into an electric signal, wherein the SiGe-based thin film is produced by heat treating a SiGe-based semiconductor thin film material after sputtering vaporization; to the above-mentioned method for forming a thin film wherein the substrate temperature and/or the plasma output is raised in the formation of the SiGe-based semiconductor thin film by sputtering vaporization, to form a thin film with a more highly crystallized structure; to an SiGe-based thin film produced by the above-mentioned method, which serves as a member of a thermoelectric transducing material component that is a constituent element of a sensor device whose signal source is a temperature differential and that transduces a local temperature differential into an electric signal, and which has been endowed with good thermoelectric characteristics by heat treatment; and to a gas sensor device containing as a constituent element the above-mentioned SiGe-based thin film.

TECHNICAL FIELD

This invention relates to an SiGe-based semiconductor thin film to beapplied in electronic devices such as thin film transistors used forhigh-speed operation at high frequency region, and to a method formanufacturing this thin film, and further to a semiconductor deviceusing this semiconductor thin film. The SiGe-based thermoelectrictransducing thin film material of the present invention is useful as amember for thermopiles, and more specifically, is used in sensors thatdetect as a voltage signal a change in a local temperature differentialcaused by heat generated in the catalytic reaction of a catalystmaterial, and is useful as a gas sensor or a similar device whose signalsource is a local increase in temperature, such as an infrared sensor,for example.

BACKGROUND ART

In a device in which slight temperature changes or tiny amounts ofthermal energy is detected, a sensor that works by transducing atemperature differential generated from a signal source into anelectrical signal is used. In this kind of sensors, there is athermopile type, which detects a temperature change asthermoelectromotive force by utilizing the Seebeck effect of athermocouple or a thermopile, which consists of a plurality of thesethermocouples connected in series. Other known types of device thatdetect temperature changes include a pyroelectric type, which detects achange in a floating charge produced by polarization corresponding tothe thermal energy of infrared rays in a base material made of ceramicor the like (this type utilizes the pyroelectric effect), and a systemwhich detects a change in resistance produced by the heat of atemperature-sensitive resistor formed from ultrafine wire or a thin filmof metal or the like (this system utilizes resistance changes) [K.Matsui, Sensa Katsuyou 141 no Jisseki Nouhau (Specific applications ofactual results and know-how of 141 uses of sensors), chapter 2, CQPublishing, 2001].

Of these, thermoelectric transducing devices that utilize the Seebeckeffect are commonly used in infrared sensors, for example, because theyare best for measuring temperature or for monitoring temperaturedifferentials. The thermoelectric transducing material thin film(hereinafter referred to as thermoelectric thin film) used for thesethermoelectric transducing devices is usually what is known as ametal-based thermoelectric semiconductor, which exhibits highelectroconductivity and has a high Seebeck coefficient, such as bismuth(Bi), tellurium (Te), or antimony (Sb) (see, for example, JapaneseLaid-Open Patent Publication No. 2000-292254).

These materials, however, are highly toxic, and furthermore there aremany limitations on their film formation and working processes. In thecase of the above-mentioned metal-based thermoelectric thin filmmaterials, it is difficult to etch the film after being formed into athin film, and it is no easy task to form a pattern by a process such aslift-off. Actually, the most common approach with these materials is toform a thin film directly by vapor deposition through a metal mask. Withthis process, though, it is difficult to perform finer working, andlimits of width of a line to be processed make it difficult to raise thedegree of integration thereof.

Similarly, SiGe is an example of a material that exhibits highthermoelectric transducing efficiency while also being easy to processand having low toxicity. SiGe-based thermoelectric materials have a longhistory of application, including use as a thermoelectric material inthe space development, and in more recent years semiconductor thin filmsbased on SiGe alloys have been widely used as members for devices to beused at high-temperature operations and for devices used in high-speedcommunications.

Known methods for manufacturing an SiGe thin film include a method inwhich hydrogen or GeF₃ is mixed into silane (SiH₄) gas, and a thin filmis deposited by vacuum CVD or plasma CVD while being crystallized, and amethod in which an amorphous thin film is formed on a substrate as anamorphous precursor, and this thin film is then crystallized. The formermethod, in which a deposited thin film is crystallized, promotescrystallization simultaneously with the formation of the thin film, butits drawbacks include the high cost of the processing equipment and theneed to subject the substrate itself to a relatively high temperature of600° C. or higher. Solid-phase growth method in which annealing isperformed over an extended period is known as a type of the lattermethod in which an amorphous silicon thin film is first formed and thencrystallized, but this method is impractical because it takes so long,and another drawback is higher manufacturing cost.

Also, when CVD is employed for forming a crystalline or amorphoussemiconductor thin film, since the film contains about 2 to 20 at %hydrogen, an annealing treatment in an electric furnace is necessary toremove the hydrogen gas from the film. This process requires thatannealing for degasification be performed at high temperature for anextended period, and this hampers efforts at increasing productivity,and the heat involved in the degasification treatment causes thesubstrate to deform, or contaminants from the substrate are diffused inthe thin film, among other such problems.

One heat treatment method involves crystallizing the material byirradiating it with an excimer laser. An amorphous thin film or apolycrystalline thin film is formed on a substrate and irradiated withan excimer laser to heat and crystallize the thin film. With thistechnique, however, it is extremely difficult to maintain a consistentcrystal quality in the thin film, and variance tends to occur in theelectrical characteristics of the manufactured thin film as well.

DISCLOSURE OF THE INVENTION

In light of the above situation, the present invention was developed inorder to solve the above problems encountered with prior art, and it isan object of the present invention to provide a method for producing anSiGe-based semiconductor thin film to serve as a member of athermoelectric transducing material component that is a constituentelement of a sensor device whose signal source is a temperaturedifferential and that transduces a local temperature differential intoan electric signal, an SiGe-based thin film which has been endowed withgood thermoelectric characteristics by this method, and a sensor device.

A further object of the present invention is to provide means forovercoming the limitations to the operating temperature resulted in aconventional device due to varieties of the gas selectivity of acatalyst generated with the operating temperature.

To solve the above problems, the present invention is constituted by thefollowing technological means.

(1) A method for producing an SiGe-based semiconductor thin film to beserved as a member of a thermoelectric transducing material componentthat is a constituent element of a sensor device whose signal source isa temperature differential and that transduces a local temperaturedifferential produced by a selective catalyst reaction into an electricsignal, comprising the steps of:

1) forming an SiGe-based semiconductor thin film over a substrate bysputtering vaporization; and

2) heat treating the SiGe-based semiconductor thin film material afterthe sputtering vaporization.

(2) The method according to (1) above, wherein the heat treatment isperformed at a temperature of from 600° C. to 1000° C.

(3) The method according to (1) above, wherein the substrate temperatureand/or the plasma output is raised in the formation of a SiGe-basedsemiconductor thin film by sputtering vaporization method, to form athin film with a more highly crystallized structure.

(4) The method according to (1) above, wherein the heat treatment isperformed by furnace annealing with a controlled atmosphere using anordinary electric furnace, or by rapid thermal process using an infraredlamp heating apparatus capable of atmosphere control.

(5) The method according to (1) above, wherein, during sputtering, athin film is produced by first doping an SiGe target with an impurity,and during heat treatment, the gas atmosphere, temperature, heattreatment duration, and temperature elevation time are controlled, sothat crystallization is performed while the amount of impurity in thesemiconductor thin film is controlled.

(6) The method according to (1) above, wherein, during heat treatment,the heat treatment conditions are controlled, an insulator thin film ofan oxide is grown over the semiconductor thin film, and crystallizationis performed while an insulation layer is produced.

(7) The method according to (1) above, wherein, during the sputteringvaporization of the SiGe-based thin film, the temperature of the heattreatment can be lowered by vapor depositing a transition metal typifiedby nickel.

(8) The method according to (1) above, wherein a sensor device whosesignal source transduces a local temperature differential produced by aselective catalyst reaction into an electric signal is exposed to avolatile organosilicon gas to form a thin film on the surface thereof,thereby increasing the gas selectivity thereof.

(9) An SiGe-based thin film produced by the method according to any of(1) to (8) above, which serves as a member of a thermoelectrictransducing material component that is a constituent element of a sensordevice whose signal source is a temperature differential and thattransduces a local temperature differential into an electric signal, andwhich has been endowed with good thermoelectric characteristics by heattreatment.

(10) A gas sensor device containing as a constituent element theSiGe-based thin film according to (9) above.

The present invention will now be described in further detail.

In the present invention, a SiGe-based semiconductor thin film is usedas a member of a thermoelectric transducing material component that is aconstituent element of a sensor device whose signal source is atemperature differential and that transduces a local temperaturedifferential into an electric signal, and thereby a high-performancesensor device can be realized. With the present invention, a higheroutput signal and lower noise are obtained, for example, when an oxidethermoelectric material is used as the thermoelectric transducing memberof this type of gas sensor device (W. Shin et al., “Thermoelectricthick-film hydrogen gas sensor working at room temperature,” Jpn. J.Appl. Phys., 40, 11B, pp. L1232-L1234, 2001). This is because thethermoelectric transducing performance of the SiGe-based material issuperior to that of an oxide.

Also, with the present invention, sputtering method is used to produce aSiGe-based semiconductor thin film. This is preferable in the productionof a device with high performance and stable characteristics, andfurthermore makes it possible to produce a satisfactory semiconductorthin film in a short time and with a simple manufacturing process.Furthermore, simultaneous patterning with a metal mask is possible,allowing the overall process to be simplified.

However, problems encountered with an SiGe thin film produced bysputtering method were that its resistance was high, the signal outputhad poor stability, and so forth. Research conducted by the inventorshas revealed that this is the reason for the poor crystallinity of thevapor deposited thin film. In view of this, in the present invention, athin film material that has relatively poor crystallinity aftersputtering vaporization is subjected to a heat treatment, which improvescrystallinity and allows the required characteristics to be imparted.

Further, with the present invention, the heat treatment can besimplified by making a thin film with increased crystallinity, even ifonly slightly, in the vaporization of the thin film, and this allows acompletely novel semiconductor thin film to be manufactured.Accordingly, with the present invention, the substrate temperatureand/or the plasma output is raised in the process of the crystallizationof an amorphous thin film, thereby a thin film with a more highlycrystallized structure is formed, even in the state immediatelyfollowing vaporization.

In the present invention, these heat treatments can be accomplished, forexample, by furnace annealing, using an ordinary electric furnace and acontrolled atmosphere. Also, a crystalline thin film that is easier tocontrol can be manufactured by employing a rapid thermal process inwhich an infrared lamp heating apparatus is used to raise thetemperature elevation rate during the heat treatment. However, themethod and means for the heat treatment are not limited to these withthe present invention.

Furthermore, crystallization can be performed while the amount ofimpurity in the thin film is controlled, and a crystalline thin film canbe manufactured, by controlling the gas atmosphere, temperature, heattreatment duration, and temperature elevation time during the heattreatment. Also, with the present invention, sputtering vaporization canbe performed with a single target if the target is an SiGe alloysemiconductor to start with, or, the target can be first doped with animpurity element prior to sputtering vaporization, and impurity elementdoping can be performed simultaneously with the formation of the filmduring thin film vaporization, allowing a doped semiconductor thin filmto be produced.

In the heat treatment, an oxide is produced on the surface of the thinfilm by the oxygen partial pressure in the atmosphere. This oxide is asilicon oxide composed of silicon and oxygen, and grows while consumingthe silicon that is a component of the SiGe thin film. The SiGe can evendisappear if the oxide film is grown in large enough quantity. In theprocess, the germanium component is purged from the silicon oxide filmand collects at the interface with the SiGe (Nayak, D K et al., Kineticsand mechanism of oxidation of SiGe: dry versus wet, Appl Phys. Lett. 73,p. 644, 1989).

The process for forming insulated thin film that is required for thewiring and so forth of devices can be omitted with the present inventionby utilizing this oxide. With the present invention, it is possible toform an insulated thin film of an oxide grown on the SiGe semiconductorthin film, and to produce this insulated layer crystallized formanufacturing a crystalline thin film by controlling the process isconditions including the atmosphere of the heat treatment. An advantageof the present invention is that the step of forming the insulated filmcan be omitted by forming an insulated film of an oxide on the surfaceof the semiconductor thin film as a semiconductor thin film formed on aninsulating substrate.

In the present invention, an SiGe-based semiconductor thin film isproduced by sputtering to be served as a member of a thermoelectrictransducing material component that is a constituent element of a sensordevice whose signal source is a temperature differential and thattransduces a local temperature differential produced by a selectivecatalyst reaction into an electric signal. Thus, with the presentinvention, an SiGe-based semiconductor thin film is formed by sputteringmethod in order to produce a satisfactory semiconductor thin film in ashort time, but since the thin film will have relatively poorcrystallinity with vaporization alone in this case, the thin filmmaterial is heat treated after the vaporization to increase itscrystallinity and endow it with the required characteristics.

This heat treatment is preferably performed by so-called furnaceannealing using an ordinary electric furnace with a controlledatmosphere. This heat treatment is performed in an argon atmosphere, forapproximately 5 to 24 hours at a treatment temperature of 700 to 1000°C. Problems such as almost no crystallization occurring will beencountered if the treatment temperature is under 700° C., but it isalso undesirable for the temperature to be over 1000° C. because thishigh-temperature process will accompany with reactions with thesubstrate and other problems. The heat treatment temperature can belowered by raising either the substrate temperature or the plasma outputduring sputtering vaporization. The effect of this is most clearlyapparent when the substrate temperature is at least 100° C. In the caseof plasma output, the effect will be pronounced from 200 W and up with a3-inch target. This method forms a thin film with a more highlycrystallized structure, even in the state immediately following thevaporization, and this method also has the effect of lowering the heattreatment by about 100° C. Therefore, the heat treatment can beperformed at 600° C. or higher in this case.

Further, with the present invention, the heat treatment duration can bereduced to no more than 30 minutes by using a rapid thermal processfeaturing an infrared lamp heating apparatus capable of controlling theatmosphere. In the heat treatment, crystallization can be performedwhile controlling the gas atmosphere, temperature, the heat treatmentduration, and the temperature elevation rate, and further controllingthe amount of impurity in the SiGe thin film. Also, an oxide producedover the SiGe semiconductor thin film after heat treatment can beutilized as an insulation layer. For example, the heat treatment isperformed such that the thickness of the insulation layer produced onthe surface of an SiGe semiconductor thin film of approximately 600nanometers will be approximately 100 nanometers. Since this film can beutilized as an insulation layer, a window is subsequently made in justthe portion where electrical contact is necessary. With the presentinvention, the SiGe thin film produced by the above method can beutilized along with a suitably catalyst material to create a suitablegas sensor device. In this case, a platinum catalyst for hydrogendetection was used as the catalyst material in the examples given below,but the present invention is not limited to this, and any suitablecatalyst material can be used.

The amount of doping of the thin film, and whether the film is n type orp type, can be controlled by controlling the gas atmosphere,temperature, heat treatment duration, and temperature elevation timeduring the heat treatment. This is because an SiGe alloy semiconductornaturally tends to be n type, and sputtering vaporization can beperformed by doping the target ahead of time with an n type impurityelement.

It is also possible to form a thin film on a substrate that is notstable at high temperature, such as glass or plastic, by lowering thetemperature of the heat treatment required for crystallization. Theaddition of a transition metal, as recently reported, is an effectiveway to further lower the crystallization temperature of an SiGe material(C. Hayzelden and J. Batstone, J. Appl. Phys., 73 (1993), 8279-8289).

With a gas sensor that utilizes a reaction at the catalyst surface, itsperformance is decreased by the production of a film of impurities orthe like on the catalyst surface. A typical example of this is thepoisoning of a catalyst by a volatile organosilicon (such ashexamethyldisilane, HMDS) gas. This volatile organosilicon lowerscatalytic activity by forming a film of silicon oxide on the catalystsurface. Nevertheless, a structure with which selective gas permeationis possible will be formed under certain film production conditions, anda selective catalyst reaction can be induced. Such a film is sometimesintentionally formed on a ceramic sensor surface. One known method forincreasing the gas selectivity of a gas sensor is to form what is knownas a molecular sieve, which is a physical filter, on the surface of thesensor material of a gas sensor by chemical vapor deposition method(CVD). (See, for example, A. Katsuki and K. Fukui, H ₂ _(—) selectivegas sensor based on SnO ₂, Sensors and Actuators B, 52, pp. 30-37(1998)).

The effects of the present invention are (1) a sensor device whosesignal source transduces a local temperature differential produced by acatalyst reaction into an electric signal is exposed to a volatileorganosilicon gas to form a thin film on the surface thereof, therebyincreasing the gas selectivity thereof, (2) the device temperature mustbe raised high in order to form a film on a catalyst surface by chemicalvapor deposition method, and performing this process adversely affectsthe properties of the catalyst, but with the present invention, a thinfilm is formed on the surface of the device by exposing the device to avolatile organosilicon gas at a relatively low temperature below 200°C., which is near the operating temperature of the device, and thisproduct is then heat treated at a higher temperature, which forms asolid film while preventing catalyst degradation, and this increases gasselectivity, (3) the gas selectivity of the device can be increased byforming a thin film on the surface of the device by exposing it to avolatile organosilicon gas, and (4) the gas selectivity can be increasedby forming a thin film on the surface of the device by exposing it to avolatile organosilicon (hexamethyldisilane, HMDS) gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the X-ray diffraction patterns of SiGe thin films producedby sputtering vaporization;

FIG. 2 is a graph of the hydrogen concentration and outputcharacteristics of an SiGe thin film in a thermoelectric hydrogen gassensor at the operating temperature;

FIG. 3 shows the surface of a sensor device observed under an electronmicroscope;

FIG. 4 is a graph of the improvement in voltage signal and devicecharacteristics as a function of heat treatment temperature;

FIG. 5 is a graph of the response characteristics of a produced sensordevice;

FIG. 6 is a graph of the results of an experiment comparing theselectivity of a produced hydrogen gas sensor for hydrogen and otherflammable gases;

FIG. 7 is a graph of the temperature dependence of the Seebeckcoefficient of an SiGe thin film;

FIG. 8 is a graph of the temperature dependence of the Seebeckcoefficient of an SiGe thin film;

FIG. 9 shows a device pattern produced on a glass substrate;

FIG. 10 shows the X-ray diffraction patterns of SiGe thin films as afunction of heat treatment temperature after sputtering vapordeposition; and

FIG. 11 shows the improvement in voltage signal and devicecharacteristics as a function of heat treatment temperature.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in more specific termsthrough examples, but the present invention is not limited in any way bythe following examples.

EXAMPLE 1

(1) Production of Gas Sensor

In this example, a gas sensor capable of measuring gas concentrationover a wide range and at high selectivity was produced by utilizing acatalyst exhibiting a selective catalytic oxidation reaction for aspecific type of flammable gas. The production procedure in one exampleof a gas sensor device to which the process of the present invention isapplied consists of production of a thermoelectric thin film bysputtering, heat treatment, and formation of a platinum catalyst film,in that order.

1) Target Production

1% phosphorus was mixed into an SiGe alloy (80% Si, 20% Ge), thismixture was pulverized to an average particle size of just a few micronsor less in a planetary ball mill, and the resulting powder was moldedand then sintered (by hot pressing) for 5 hours at 1000° C. to produce asinter. This sinter was used as a sputtering target.

2) Thermoelectric Film Production

Using the above target, a film of an SiGe thermoelectric transducingmaterial was formed with a high frequency (RF) sputtering apparatus.This sputtering was performed at a vapor deposition pressure ofapproximately 5×10⁻¹ Pa and a sputtering output of 250 W. Sputteringvaporization was conducted under these conditions for 30 minutes,forming a film of approximately 0.7 micrometer. The thickness of thisfilm was determined from direct observation of a fracture plane thereofusing an electron microscope.

3) Heat Treatment

An SiGe thin film with increased crystallinity was produced by placingthe SiGe thin film that had undergone sputtering vaporization in afurnace with an argon atmosphere and heat treating it for approximately5 hours at 900° C. In this heat treatment, the temperature, duration,and oxygen partial pressure in the atmosphere were controlled, and athin oxide was produced on the surface of the thin film under an argonflow. This oxide was silicon oxide composed of silicon and oxygen, partof which was removed with an HF-based etching solution, forming an areaof electrode contact (called a window). The window pattern was formed byphotolithography.

4) Sputtering Vaporization of Catalyst Thin Film

A catalyst thin film was formed by sputtering vaporization over part ofthe device surface that had undergone the above process. To form thisfilm in a pattern, the sputtering vaporization was performed with ametal mask placed over the device. The catalyst material here was aplatinum catalyst in order to detect hydrogen. The catalyst film wasproduced by sputtering vaporization using a platinum target and a highfrequency (RF) sputtering apparatus, at a sputtering output of 100 W for10 minutes and at a vapor deposition pressure of approximately 2×10⁻¹Pa.

5) Electrode Formation

A gold lead wire pattern was formed by sputtering vaporization using ametal mask, thereby forming wiring for signal take-off. The pattern wasproduced by sputtering vaporization using a gold target and a highfrequency (RF) sputtering apparatus, at a sputtering output of 100 W for5 minutes and at a vapor deposition pressure of approximately 2×10⁻¹ Pa.

6) Performance Evaluation

The performance of the catalyst and the device was evaluated by using aninfrared thermal camera to find the surface temperature of the thin filmcatalyst formed on the substrate. The gas used in the test was allowedto flow into the test reaction chamber at a flux of approximately 100cc/minute. Just as with the gas sensor device, changes in the surfacetemperature were measured with the infrared thermal camera under a mixedgas flow, and the output signal from the device was measured at the sametime. The thermoelectric characteristics of the SiGe semiconductor thinfilm were evaluated in the air between room temperature andapproximately 400° C. A steady-state method with high Seebeckcoefficient reliability was used as the evaluation method.

(2) Results

The effects of the present invention will now be described on the basisof the results of evaluating the characteristics of the sensor deviceproduced in the above process and its performance as a sensor.

1) Effect of Heat Treatment

An SiGe thin film that was not heat treated had poor characteristics,such as variance in its performance when the operating temperature wasraised. Specifically, a thermoelectric gas detection sensor was producedby forming a layer of platinum catalyst in a thickness of about 50nanometers with a sputtering apparatus over half of the SiGe thin filmsurface following sputtering vaporization, but the sensor deviceproduced from this thin film had high resistance and its signal outputwas inconsistent, which meant that reproducibility was poor.

The reason for this lies in the low crystallinity of a thin filmproduced by sputtering. Analysis of the X-ray diffraction patternconfirmed that the vapor deposited thin film was not crystalline, butamorphous. FIG. 1 shows this XRD pattern. An attempt to measure theresistance of the thin film was also made by four-terminal method, butthe resistance of the vapor deposited thin film was so high thatmeasurement was impossible.

Meanwhile, an SiGe thin film produced by sputtering vaporization wasplaced in a furnace with an argon atmosphere and heat treated for about5 hours near 900° C., which increased the crystallinity. As shown inFIG. 1, this can be clearly confirmed from analysis of the X-raydiffraction pattern after heat treatment. The vapor deposited thin filmwas confirmed to be not crystalline, but amorphous. In the figure, thepeaks marked with a square were produced by SiGe crystals. Those peakswith no mark were produced by the substrate. When the crystallinity waschecked while gradually raising the heat treatment temperature from 600°C., strong peaks attributable to SiGe were measured, particularly fromheat treatment at 700° C. and up. Specifically, this indicates thatcrystalline SiGe began to grow in the thin film, which was amorphous.Crystallization was substantially completed by heat treatment at 900° C.It was found that there was not much change in the degree ofcrystallization when heat treatment was continued for a long time atthis temperature. Conversely, if the heat treatment was continued toolong, it was found that the process of oxidation began even at the lowoxygen partial pressure present in the argon atmosphere, and theproduction of silica and SiO₂ began. The peaks near 22 degrees markedwith a circle in the figure were produced by silica.

2) Evaluation of Sensor Device

A sensor was produced by vapor depositing platinum (serving as thecatalyst) in a thickness of several dozen nanometers over half of athick film surface, and this catalyst film was formed with a sputteringapparatus, just as with SiGe. The sputtering was performed at a vapordeposition pressure of approximately 4×10⁻² Torr and a sputtering outputof 100 W for 5 minutes. FIG. 2 shows the hydrogen detectioncharacteristics of this sensor. It can be seen that very linear outputcharacteristics were obtained versus hydrogen concentration. The voltageoutput rises with operating temperature, but levels off at hightemperature. These characteristics are dependent on the characteristicsof the platinum catalyst.

FIG. 3 shows an electron micrograph of the surface of a sensor deviceconsisting of SiGe and platinum catalyst. It can be seen that the SiGethin film has a particle structure and forms a solid film. The degree ofcrystallinity of this film, that is, how much the film is crystallized,affects the ultimate device characteristics. FIG. 4 shows therelationship between heat treatment temperature and voltage signal. Itcan be seen that crystallization increases with the heat treatmenttemperature, and as a result, the device characteristics tend to beimproved. A comparison of the crystallization results in FIG. 1 revealsthat crystallization increases and a stable sensor signal output isobtained when the heat treatment temperature is raised.

FIG. 5 shows the response characteristics of the sensor device. FIG. 6shows the results of an experiment in which the hydrogen selectivity asa hydrogen sensor was compared to that for other flammable gases. Inprinciple, higher selectivity should indeed be obtained when platinum isused as the catalyst, and as shown in the graphs, at about 150° C. orlower there was almost no response to gases other than hydrogen. Thisproves that a thermoelectric hydrogen sensor made from SiGe functionsadequately as a hydrogen sensor.

3) Simultaneous Production of Oxide Film by Heat Treatment

Even though the heat treatment was performed while argon gas was allowedto flow into an electric furnace at about 100 ccm, a small amount ofoxygen partial pressure was still present in the furnace. This oxygenreacted with the SiGe at high temperature, forming silica, a thin filmof silicon oxide on the surface of the SiGe thin film. Because this isan insulator through which electricity cannot flow, this silica must beremoved when an SiGe thin film is used. With the present invention,however, only the part of this insulation layer that is required forelectrical contact is removed, rather than the entire layer, and awindow measuring 60 microns square was provided for this purpose.

The etching for this window was performed for approximately 60 secondsusing a 1:6:4 solution of HF:H₂O:NH₄F, which is what is ordinarily usedfor etching silica. As a result, the etching solution removed just thewindow portion of the silica. After this, an electrode pattern wasformed and electrical resistance was measured, which confirmed that agood electrical connection had been formed on the SiGe surface. Thistells us that an oxide film produced simultaneously during thecrystallization process can be effectively utilized as an insulationlayer.

4) Change in Crystallization Due to Sputtering Conditions

A problem with using a high temperature during heat treatment is that itcan make other processes difficult. In an effort to lower thistemperature or make heat treatment unnecessary, an attempt was made tocreate a thin film with somewhat higher crystallinity during sputteringvapor deposition. Three variables of the process conditions, namely,distance between the substrate and the target, argon gas flux, and vapordeposition time, did not greatly affect crystallization. However, thecrystallization of the thin film changed markedly when the plasma outputin sputtering was raised over 200 W. When an SiGe thin film was vapordeposited for 30 minutes at 250 W, an SiGe peak was confirmed from theX-ray diffraction pattern even without any heat treatment. When heattreatment was performed after this, crystallization proceeded evenfurther, and peak intensity rose. The SiGe thin film vapor deposited ata high output of about 250 W had much higher crystallinity even at a lowheat treatment temperature of 700° C., the result being the effect oflowering the heat treatment temperature by about 100° C. or more. It wasalso found that a similar effect is obtained either by raising thesubstrate temperature of raising the sputtering plasma output.

5) Process by which Doping Amount can be Controlled (Part 1)

When an SiGe target is pre-doped with an impurity, a problem is that thedoped component, such as impurity phosphorus, is evaporated by the heattreatment, which markedly reduces the doping amount, but if the heattreatment temperature can be lowered, or if the device can be useddirectly without any heat treatment at all, then the doped component canbe left intact. FIG. 7 shows the temperature dependence of the Seebeckcoefficient of the sample in an example of this. The sample was producedby vapor deposition for 40 minutes at a sputtering output of 250 W and asubstrate temperature of 300° C., using a phosphorus-doped SiGe target.

The thin film was then subjected to heat treatment for 5 hours at 900°C. while argon was allowed to flow through an ordinary electric furnace.As shown in FIG. 7, at a low temperature, the effect of doping was thatthis product exhibited n type characteristics, the Seebeck coefficientwas negative, and the main charge carrier of the sample was electrons.

Meanwhile, when the evaporation of phosphorus was aided by extending theheat treatment duration, the amount of residual phosphorus decreased,and the sample became p type. A characteristic of this sample was thatwhen the temperature was raised, the charge carrier became holes, andthe sign of the Seebeck coefficient inverted to positive.

6) Process by which Doping Amount can be Controlled (Part 2)

When an SiGe target is pre-doped with an impurity, a problem is that thedoped component (phosphorus) is evaporated by the heat treatment, whichmarkedly reduces the doping amount, but if the vapor depositionconditions, such as the substrate temperature during sputteringvaporization, and the heat treatment conditions, such as the heattreatment temperature, are varied, then the doped component can be leftintact. FIG. 8 shows the temperature dependence of the Seebeckcoefficient of the sample in an example of this. The sample was producedby vapor deposition for 40 minutes at a sputtering output of 250 W and asubstrate temperature of 200° C., using a phosphorus-doped SiGe target,performed for 5 hours at 800° C. while argon was allowed to flow throughan ordinary electric furnace. As shown in FIG. 8, since the doped amountof phosphorus was sufficient, the Seebeck coefficient was negative overthe entire temperature range, and n type characteristics were exhibited.

EXAMPLE 2

(1) Production of Gas Sensor

In this example, the device was the same as the gas sensor in Example 1,but the design thereof was different, and in particular, a heater linewas formed from platinum, and a mechanism for heating the device wasformed simultaneously. The process was basically the same as that inExample 1, but differed in the following points. 1) A process in whichnickel (a transition metal) was simultaneously sputtered was added inthe sputtering vaporization of the SiGe. 2) Titanium (a transitionmetal) was formed as a buffer layer in order to increase adhesion to thesubstrate in the vapor deposition of the gold electrode pattern and thevapor deposition of the platinum heater. FIG. 9 shows the design of thedevice.

1) Thermoelectric Film Production

A film of a thermoelectric transducing material was formed under thesame high frequency (RF) sputtering conditions as in Example 1, and athin film of an SiGe thermoelectric transducing material was producedover glass. 7059 glass made by Corning was used as the substrate. Nickelwas vapor deposited in a thickness of about 30 nanometers prior to thesputtering of the SiGe. The other conditions were basically the same asin Example 1.

2) Heat Treatment

The SiGe thin film produced by sputtering vaporization was placed in afurnace with an argon atmosphere and heat treated for about 6 hoursbetween 500 and 600° C., which produced an SiGe thin film with increasedcrystallinity.

3) Heater Formation

After this, a heater line was formed by vapor depositing platinum overthe above product. Titanium (a transition metal) was formed in athickness of 50 nanometers as a buffer layer in order to increaseadhesion to the substrate prior to the vapor deposition of platinum. Thethickness of the platinum heater was approximately 1 micron. With thegold serving as an electrode, just as with the platinum, titanium (atransition metal) was formed in a thickness of 50 nanometers as a bufferlayer.

(2) Results

The effects of the present invention will now be described on the basisof the results of evaluating the characteristics of the sensor deviceproduced in the above process and its performance as a sensor.

1) Change in the Crystallization Temperature

FIG. 10 shows the X-ray diffraction patterns of SiGe thin films as afunction of heat treatment temperature after sputtering vaporization. Itcan be seen that when nickel was vapor deposited, the temperature of theheat treatment required for crystallization was several hundred degreescentigrade lower than in Example 1. This makes it clear that this is aparticularly effective method for glass substrates that cannot besubjected to heat treatment at high temperatures.

2) Evaluation of Sensor Device

A device produced using a glass substrate with the patterns shown inFIG. 9, produced using the same catalyst production process as inExample 1, was evaluated for response characteristics with respect to ahydrogen concentration of 3% at an operating temperature of 100° C. FIG.11 shows the improvement in voltage signal and device characteristics asa function of heat treatment temperature. In Example 1, heat treatmentat a high temperature of about 900° C. was necessary in order toreproduce adequate sensor response characteristics, but in this caseadequately high sensor output was obtained even at a temperature as lowas 550° C. Because a glass substrate was used, the heater power neededto maintain a sensor operating temperature of 100° C. was only abouthalf that in Example 1.

EXAMPLE 3

This is an example of a manufacturing method that increases gasselectivity by forming a thin silicon oxide film on the surface of thegas sensor of Example 1.

(1) Production of Gas Selective Layer Formed on Catalyst Surface

1) Poisoning with HMDS

The gas selectivity of a sensor device can be increased by exposing asensor device whose signal source converts a local temperaturedifferential produced by a catalyst reaction into an electric signal toa volatile organosilicon gas to form a thin film on the surface thereof.The device temperature must be raised to a high temperature in order toform a film by chemical vapor deposition on a catalyst surface, but aproblem with performing this process is that it adversely affects theproperties of the catalyst. The thermoelectric gas sensor device usedhere was produced by the process of Example 1. After platinum catalystvapor deposition, this device was placed in a sample treatment box witha 1000 ppm HMDS (volatile organosilicon) atmosphere, and the sample waspoisoned for 3 days while the device operating temperature was held at160° C. This poisoning process diminished the hydrogen responsecharacteristics of the device by about half of their initial level.

2) Restorative Heat Treatment and Surface Analysis

The device was then subjected to heat treatment for 2 hours at 400° C.while argon was allowed to flow through an ordinary electric furnace.This restored the hydrogen response characteristics. Since a toughsilicon oxide layer is produced over the platinum surface if the devicetemperature is high, there was no further restoration of responsecharacteristics after subsequent heat treatment. XPS analysis resultsrevealed that silicon oxide is what was formed on the surface. Inparticular, it was found that with a sample pronounced restorationcharacteristics, the O1s and Si2p contents were lower than beforepoisoning, and a layer having chemical bonds of oxygen and silicon onthe platinum surface was removed by heat treatment.

(2) Evaluation of Sensor Device

With a poisoned sample having an extremely thin silicon oxide film, thisfilm had the effect of suppressing reactions with gases other thanhydrogen, and even when the operating temperature was as high as 160°C., the S value for hydrogen selectivity, particularly with respect tolarge gas molecules such as ethanol or methanol, was higher than with anunpoisoned sample having no silicon oxide film. The curve of responsecharacteristics was basically the same as that in Example 1 or 2.However, the signal output varied further with the type of gas beforethe poisoning treatment and following the restoration treatmentperformed after poisoning treatment.

Table 1 lists the voltage output of the sensor with respect to ahydrogen concentration of 3% at a device operating temperature of 160°C. The selectivity S is the relative size of the signal when the outputwith respect to hydrogen gas is set at 1. Accordingly, hydrogen has an Svalue of 1, and this is not shown in Table 1. First, with a device thatunderwent everything up to the restoration treatment, the voltage signaldecreased to 87% of what it had been prior to poisoning. However,hydrogen selectivity was improved by this treatment. When the deviceoperating temperature was high, flammable gases such as methanol orethanol were prone to catalytic combustion, and became a problem asinterfering gas, but because of this molecular sieve-like surface layer,these reactions were suppressed, and sensor output for these largermolecules decreased. For example, the output was 6.5 times less in thecase of methanol, and was 6.2 times less in the case of ethanol. Becausethere was little decrease for hydrogen, the hydrogen selectivity Simproved markedly. TABLE 1 Conditions H₂ CO CH₄ i-C₄H₁₀ C₂H₅OH CH₃OHPoisoning Vs Vs S Vs S Vs S Vs S Vs S Before 7.871 0.076 104 0.126 630.282 28 0.459 17 0.673 11 After 6.865 0.059 116 0.108 64 0.106 65 0.07493 0.103 67

INDUSTRIAL APPLICABILITY

As discussed in detail above, the present invention relates to anSiGe-based thin film, to a method for manufacturing this film, and to asensor device. With the present invention, it is possible to produce, bysputtering, an SiGe-based semiconductor thin film that has excellentthermoelectric performance as a member of a thermoelectric transducingmaterial component that is a constituent element of a sensor devicewhose signal source is a temperature differential and that transduces alocal temperature differential into an electric signal. Also,crystallinity can be increased and the required characteristics impartedby heat treating the thin film material after SiGe vapor deposition.Further, the temperature of the heat treatment can be lowered and thetreatment duration shortened by changing the process conditions insputtering vaporization and performing rapid heating. Theelectroconductivity of a thermoelectric transducing thin film materialcan be controlled by controlling the heat treatment conditions. An oxidethin film produced during heat treatment can be utilized as theinsulation layer required for device production. The temperature of heattreatment required for SiGe crystallization can be lowered byintroducing nickel during sputtering vaporization, allowing a thin filmto be formed on a substrate that is not stable at high temperatures,such as glass or plastic. Also, hydrogen gas selectivity can beincreased by forming a thin layer like a molecular sieve from volatileorganosilicon on a catalyst surface, and controlling the film productionconditions and so forth.

1. A method for producing an SiGe-based semiconductor thin film to beserved as a member of a thermoelectric transducing material componentthat is a constituent element of a sensor device whose signal source isa temperature differential and that transduces a local temperaturedifferential produced by a selective catalyst reaction into an electricsignal, comprising the steps of: (1) forming an SiGe-based semiconductorthin film over a substrate by sputtering vaporization; and (2) heattreating the SiGe-based semiconductor thin film material after thesputtering vaporization.
 2. The method according to claim 1, wherein theheat treatment is performed at a temperature of from 600° C. to 1000° C.3. The method according to claim 1, wherein the substrate temperatureand/or the plasma output is raised in the formation of a SiGe-basedsemiconductor thin film by sputtering vaporization method, to form athin film with a more highly crystallized structure.
 4. The methodaccording to claim 1, wherein the heat treatment is performed by furnaceannealing with a controlled atmosphere using an ordinary electricfurnace, or by rapid thermal process using an infrared lamp heatingapparatus capable of atmosphere control.
 5. The method according toclaim 1, wherein, during sputtering, a thin film is produced by firstdoping an SiGe target with an impurity, and during heat treatment, thegas atmosphere, temperature, heat treatment duration, and temperatureelevation time are controlled, so that crystallization is performedwhile the amount of impurity in the semiconductor thin film iscontrolled.
 6. The method according to claim 1, wherein, during heattreatment, the heat treatment conditions are controlled, an insulatorthin film of an oxide is grown over the semiconductor thin film, andcrystallization is performed while an insulation layer is produced. 7.The method according to claim 1, wherein, during the sputteringvaporization of the SiGe-based thin film, the temperature of the heattreatment can be lowered by vapor depositing a transition metal typifiedby nickel.
 8. The method according to claim 1, wherein a sensor devicewhose signal source transduces a local temperature differential producedby a selective catalyst reaction into an electric signal is exposed to avolatile organosilicon gas to form a thin film on the surface thereof,thereby increasing the gas selectivity thereof.
 9. An SiGe-based thinfilm produced by the method according to any of claims 1 to 8, whichserves as a member of a thermoelectric transducing material componentthat is a constituent element of a sensor device whose signal source isa temperature differential and that transduces a local temperaturedifferential into an electric signal, and which has been endowed withgood thermoelectric characteristics by heat treatment.
 10. A gas sensordevice containing as a constituent element the SiGe-based thin filmaccording to claim 9.